The dimensions of semiconductor device features relentlessly plunge into the deep sub-micron range challenging conventional fabrication capabilities. As critical dimensions shrink, it becomes increasingly more difficult to achieve high dimensional accuracy in an efficient manner with high manufacturing throughput. The minimum feature size depends upon the chemical and optical limits of a particular lithography system, and the tolerance for distortions. In addition to the limitations of conventional lithography, as dimensions shrink and memory capacity increases, manufacturing costs increase, thereby requiring advances in processing aimed at the efficient use of facilities and high manufacturing throughput. In today's competitive market, a yield of at least 70% is required for profitability.
It has been recently proposed to employ shorter wavelength radiation, e.g., of about 3 to 20 nm. Such radiation is conventionally termed EUV or soft X-ray. Sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. Such EUV lithography exposure tools, however, have been problematic. A major source of concern is the contamination of the reflective optical elements during operation due to residual gases from the vacuum atmosphere.
Optical reflective elements for the soft X-ray to EUV wavelength range, such as photomasks or multilayer mirrors, are required for use in EUV lithography of semiconductor components. Typical EUV lithography exposure devices have eight or more reflective optical elements. In order to achieve a sufficient overall intensity of the working radiation, the mirrors must have the highest possible reflectivities, since the overall intensity is proportional to the product of the reflectivities of the individual mirrors. These high reflectivities should be retained by the reflective optical elements if possible throughout their lifetime. Furthermore, the homogeneity of the reflectivity across the surface of the reflective optical element must be preserved for the entire lifetime. The reflectivity and the lifetime of these reflective optical elements are especially impaired by surface contamination in the form of carbon deposits and by surface oxidation during exposure to the operating wavelength.
The reflective optical elements are contaminated during operation by residual gases in the vacuum atmosphere. A contamination mechanism comprises adsorption of residual gases on the surfaces of the reflective optical elements. The adsorbed gases are broken up by the high-energy photon radiation through emission of photoelectrons. When hydrocarbons are present in the residual gas atmosphere, a carbon layer is thus formed, which diminishes the reflectivity of a reflective optical element by around 1% per nm of thickness. At a partial pressure of hydrocarbons of around 10−9 mbar, a layer of 1 nm thickness will be formed already after around 20 hours. Commercial specifications for projection optics lifetime is a reflectance loss of less than 1% per surface. A 15 Å thick film of carbon in the form of graphite would reduce the reflectivity of an EUV optic by 2%. Since, for example, EUV lithography devices with a reflectivity loss of 1% per reflective optical element no longer allow the necessary production pace, this contamination layer must be removed by a cleaning process which typically takes up to 5 hours, thereby reducing manufacturing throughput. Moreover, such cleaning is likely to damage the surface of the reflective optical element, as by roughening or oxidizing the surface, thereby preventing the initial reflectivity from being regained.
Conventional approaches to the carbon deposition problem also include the use of EUV imaging optics comprising multilayers of various elemental combinations deposited on glass substrates. Silicon has historically been chosen for the final capping layer because it is less susceptible to oxidation than molybdenum when exposed to air. However, silicon is susceptible to rapid EUV-induced oxidation by water vapor. Ruthenium (Ru) has shown promise as an alternative capping layer, because it is less sensitive to radiation-induced oxidation. However, Ru-capped multilayers are susceptible to carbon deposition. To ensure maximum EUV exposure, it is imperative that EUV optics be maintained as clean as possible during operation.
Accordingly, a need exists for lithographic exposure tools, particularly EUV lithography exposure tools, with reduced carbon contamination on the reflective elements during use. There also exists a need for methodology enabling careful control of the hydrocarbon level such that it is sufficiently high enough to protect the fragile multilayer reflective layers from oxidation, but low enough to avoid carbon build up on the optical elements due to cracking of the hydrocarbons by EUV radiation.